Unsaturated fatty acids such as linoleic (C.sub.18 .DELTA..sup.9,12) and .alpha.-linolenic (C.sub.18 .DELTA..sup.9,12,15) acids are essential dietary constituents that cannot be synthesized by vertebrates since vertebrate cells can introduce double bonds at the .DELTA..sup.9 position of fatty acids but cannot introduce additional double bonds between the .DELTA..sup.9 double bond and the methyl-terminus of the fatty acid chain. Because they are precursors of other products, linoleic and .alpha.-linolenic acids are essential fatty acids, and are usually obtained from plant sources. Linoleic acid can be converted by mammals into .gamma.-linolenic acid (GLA, C.sub.18 .DELTA..sup.6,9,12) which can in turn be converted to arachidonic acid (20:4), a critically important fatty acid since it is an essential precursor of most prostaglandins.
The dietary provision of linoleic acid, by virtue of its resulting conversion to GLA and arachidonic acid, satisfies the dietary need for GLA and arachidonic acid. However, a relationship has been demonstrated between consumption of saturated fats and health risks such as hypercholesterolemia, atherosclerosis and other chemical disorders which correlate with susceptibility to coronary disease, while the consumption of unsaturated fats has been associated with decreased blood cholesterol concentration and reduced risk of atherosclerosis. The therapeutic benefits of dietary GLA may result from GLA being a precursor to arachidonic acid and thus subsequently contributing to prostaglandin synthesis. Accordingly, consumption of the more unsaturated GLA, rather than linoleic acid, has potential health benefits. However, GLA is not present in virtually any commercially grown crop plant.
Linoleic acid is converted into GLA by the enzyme .DELTA.6-desaturase. .DELTA.6-desaturase, an enzyme of about 359 amino acids, has a membrane-bound domain and an active site for desaturation of fatty acids. When this enzyme is transferred into cells which endogenously produce linoleic acid but not GLA, GLA is produced. The present invention, by providing the gene encoding .DELTA.6-desaturase, allows the production of transgenic organisms which contain functional .DELTA.6-desaturase and which produce GLA. In addition to allowing production of large amounts of GLA, the present invention provides new dietary sources of GLA.